Phenylacetate is a natural compound which has a profound effect on tumor growth. In collaboration with Drs. Samid and Myers of the NCI, we evaluated the in vivo effect of phenylacetate on prevention of tumor growth and antitumor efficacy against established tumors in an animal model. Phenylacetate induced differentiation of the malignant tumor cells and extended survival when given simultaneously with tumor inoculation or as treatment to established tumors. No toxicity was associated with the therapy and therapeutic drug levels were achieved in the plasma and CSF. Electron microscopy of treated tumors showed striking hyperplasia of the rough endoplasmic reticulum as an in vivo marker of tumor differentiation. In vivo proliferation assays demonstrated a significant decrease in the mitotic index of treated tumors.